This invention relates to a novel orthodontic system and orthodontic method for the orthodontic movement of malaligned teeth and more particularly to a system utilizing an ultraelastic material which applies a variable orthodontic load in response to temperature change.
Conventional systems for the orthodontic movement of teeth have usually been based on the elasticity of a metal wire. A load created by bending a metal wire is applied to the tooth to be corrected in order to move it in the direction of the load. Such conventional systems utilize orthodontic metal wires formed from stainless steel, a Co-Cr based alloy or an intensly worked Ni-Ti alloy. The elasticity of these wires is represented by the proportional elastic limit of the metal or alloy involved. The Ni-Ti alloy has a higher proportional elastic limit than the stainless steel or other alloys. However, the Ni-Ti alloy does not exhibit elongation exceeding about 2% in a tensile test. The stainless steel and other alloys show elongation which is less than 1%. Thus, if the wire is bent or pulled beyond its proportional elastic limits it undergoes plastic deformation. Excessive deformation is unpredictable in the conventional wires. This small proportional elastic limit means that the orthodontic effect obtained is relatively small.
The elasticity of the conventionally utilized metallic materials is an inherent property thereof which is difficult to modify by heat treatment or otherwise. Specifically, it is not possible to heat treat an intensively worked Ni-Ti alloy, since heat treatment reduces by about one-half the proportional elastic limits which has been obtained by the intense working. U.S. Pat. No. 4,037,324 utilizes one such alloy wherein a stoichiometric alloy of Ni and Ti, specifically the atomic ratio of Ni to Ti is 1:1 . An orthodontic system based on this alloy presents practical problems when utilized in that a patient undergoes intense pain, often with the dental periosteum suffering from interruption in blood circulation. This occurs because the transformation temperature (which is the lowest temperature wherein the ultraelastic effect occurs) is 26.7.degree. to 32.2.degree. C., a temperature lower than normal body temperature. Thus, the large load is applied continually to the teeth at all times that the orthodontic system is disposed in the patient's mouth. This system based on the stoichiometric alloy of Ni and Ti is not fully satisfactory from the point of view of the patient's comfort.
Accordingly, it would be desirable to provide an orthodontic system which overcomes the shortcomings of the prior art systems. Such a system would apply a very small load to the teeth when the orthodontic system is disposed in a patient's mouth, and only apply an increase load when the temperature in the mouth is increased by taking in a material having a temperature above body temperature.